1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly to an arrangement for lifting a sheet supporting member having sheets stacked thereon so as to press the sheets against a feed roller.
2. Description of the Related Art
Today, among image forming apparatuses such as a copy machine, a printer, a facsimile machine and the like, widely used types of image forming apparatus feed a sheet from a sheet feeding apparatus to an image forming portion to form an image. Here, the sheet feeding apparatus generally has a sheet cassette serving as a sheet storage portion and being attachable to and detachable from the body of the apparatus, and automatically feeds a sheet stored in the sheet cassette to the image forming portion.
For example, there is known sheet cassette arranged to be able to lift and lower a sheet supporting member which has sheets stacked thereon and when the sheets are fed, is lifted to press the stacked sheets against a feed roller. When the sheets are fed, the feed roller rotates to feed the sheets pressed by the sheet supporting member to the image forming portion, one by one from the uppermost sheet.
Such a sheet feeding apparatus has a cam provided coaxially with the feed roller so as to lift and lower the sheet supporting member. When the apparatus is in a standby state and is not feeding a sheet, the sheet supporting member is forced down to a certain position by the cam, whereby the sheets are easily set and replaced. In the course of feeding out the sheets, the cam presses down the sheet supporting member, whereby a feed force by the feed roller is not applied to the sheets to be fed out. This can improve sheet separability in a separation unit such as a separation pad.
The sheet feeding apparatus of such a configuration requires one vertical reciprocating motion of the sheet supporting member for each rotation of a drive shaft, which imposes a problem in that it is difficult to reduce the size (outermost diameter) of the cam. For example, in a case in which the sheet supporting member is provided rotatably in a vertical direction, a reduction in size of the cam reduces a rotational angle of the sheet supporting member, thereby reducing the spacing between the feed roller and the sheet supporting member.
Such a reduction in the spacing between the feed roller and the sheet supporting member reduces the number of sheets to be stacked on the sheet supporting member. Then, the user has to frequently replenish sheets, thereby causing a problem of poor operability. Since there is a trade-off between the size of the cam and the rotational angle of the sheet supporting member, it is difficult to reduce the size of the cam.
As the conventional sheet feeding apparatus for the purpose of reduction in size of the cam and noise thereof, there has been proposed an arrangement of providing a lever member serving as a link member between the cam and the sheet supporting member for performing rotation. According to Japanese Patent Application Laid-Open No. 2008-105790, first and second arm portions are disposed in a lever member disposed in the apparatus body, wherein the first arm portion is in sliding contact with a cam fixed to a drive shaft of a feed roller and the second arm portion is in sliding contact with the front end portion of the sheet supporting member. According to U.S. Patent Publication No. 2008/0101837 as another arrangement, an end portion of the lever member rotatably provided in the apparatus body is locked to the sheet supporting member and an intermediate portion of the lever member is in sliding contact with the cam fixed to the drive shaft of the feed roller.
Such conventional sheet feeding apparatuses can reduce the size of the cam, but has a problem in that it is difficult to reduce the entire size of the sheet feeding apparatus under the constraints of fulcrum placement of a lever member abutting against the cam and serving as a link member so as to transmit a force to the sheet supporting member. For example, the arrangement disclosed in Japanese Patent Application Laid-Open No. 2008-105790 is such that the second arm portion of the lever member is in sliding contact with the front end of the sheet supporting member so as to move the sheet supporting member against a biasing force of a feed spring. Thus, the second arm portion has a smooth relatively long arcuate shape. In a case in which the second arm portion has such a shape, the second arm portion swings on the underside of the separation unit, whereby it is necessary to provide a space for the second arm portion to swing. Thus, the above arrangement can reduce the outermost diameter of the cam, but may increase the entire size of the sheet feeding apparatus.
The arrangement disclosed in U.S. Patent Publication No. 2008/0101837 requires an end portion of the lever member to be locked to the sheet supporting member and an intermediate portion thereof to be in sliding contact with the cam. This arrangement imposes constraints on the place where the turning fulcrum of the lever member can be positioned between the rotational trajectory of the cam and the rotational trajectory of the sheet supporting member. Further, the turning fulcrum of the lever member is projected toward a downstream side surface of the feed roller, whereby this arrangement can reduce the outermost diameter of the cam, but may increase the entire size of the sheet feeding apparatus. An increase in the entire size of sheet feeding apparatus increases the entire size of the image forming apparatus housing the sheet feeding apparatus, whereby this arrangement imposes a problem of enlarging the installation place of the image forming apparatus.